1. Field of the Invention
This invention relates to nuclear reactor cores and more particularly to core fuel assemblies which include fuel rods laterally positioned by spacing structures or grids along the assembly length.
2. Description of the Prior Art
Most liquid cooled nuclear reactors include fuel assemblies comprising nuclear fuel contained in a multiplicity of elongated cladding tubes to form fuel rods. The cladding is typically of stainless steel or zirconium alloy material, or other materials which have a relatively low neutron absorption cross section. The rods may be as long as 10 to 16 feet or more in length. To maintain proper lateral alignment among the plurality of parallel and coextending fuel rods in each fuel assembly, spacing devices are used. The spacing devices utilized typically include wire wrapping of the rods or grid structures positioned at preselected positions along the assembly length. Although many varying grid types exist, they generally comprise an "egg-crate" type structure, through which the fuel rods are inserted. The number of grids utilized may vary with the design of individual fuel assemblies. The spacing structures, or grids, also function to alleviate rod-to-rod contact among rods of the same assembly and also among rods of adjacent assemblies. Typical grid structures are described and shown in U.S. Pat. Nos. 3,379,617 and 3,379,619, both issued in the name of H. N. Andrews et al.
There is a strong economic and efficiency incentive to minimize the lateral surface area and the grid material in any one assembly. The grids, even though typically comprised of a low neutron absorbing material, such as alloys of zirconium, do absorb neutrons and detract from reactor efficiency. Minimizing this neutron poisoning effect lowers the cost of electric power. With increasing reactor fuel technology, and increased fuel operating experience, fuel assembly designs may change to lessen or increase the number of grids incorporated in a fuel assembly in a given core. For example, fuel in operating reactors today that contains seven grids along the length of the fuel assembly may, in the near future, be designed with eight or nine grids, or less than seven grids. As the grids are spaced along the assembly length at optimum structural or reactivity locations, the elevation of grids in a seven grid assembly will necessarily be different than the elevation of at least some of the grids in an eight grid assembly.
It is critical to ensure that any contact among assemblies is of a grid-to-grid variety, as opposed to rod-to-rod or rod-to-grid contact. If grid-to-grid contact is not maintained, there is a likelihood or fretting damage or coolant flow starvation at the contact point of a rod with another rod, or another grid. This flow starvation could create a local hot spot leading to local rod melting. This further could lead, in the extreme case, to formation of a hole in the rod cladding thereby allowing reactor coolant to contact the fuel and also allowing the nuclear fuel and fission products to enter the reactor coolant. Any coolant exposed to the reactor fuel creates concerns regarding radiation levels within the plant and potential environmental releases. Also, unmatched elevations of grids in adajcent assemblies can cause local coolant flow starvation and increased assembly vibration, as a result of induced coolant cross-flow. Coolant passing through an assembly tends to discharge radially upon approaching a flow restriction such as a grid. If there is no grid, or other spacing structure, at the corresponding elevation of an adjacent assembly, this may result in local flow starvation at a point just above the grid in the assembly from which the coolant is discharging radially, and, due to the high coolant velocity, also vibrate the adjacent assembly. Further, one of the most critical factors in the design of a reactor core is the spacing among the fuel rods and the fuel assemblies. Improper spacing, through improper grid-to-grid contact, could also lessen the efficiency of reactor operation or create local high power areas in the core. In addition to potential rod failure due to local hot spots, the high axial velocity of reactor coolant within the reactor core tends to vibrate the fuel rods. If proper lateral alignment is not maintained by the grid structures, the rods may sporadically contact the rods or grids of an adjacent assembly, which could lead to fretting over the period of reactor operation. The eventual result may similarly be the entrance of the fuel or fission products into the reactor coolant. The aligned grids may also perform similar functions under assumed accident conditions, such as seismic loading.
For these reasons, it is imperative that grid-to-grid contact be maintained among adajcent assemblies, while minimizing the amount of neutron poisonous or parasitic material in the core. To assure this, apparatus is required during the operating cycles when the changeover from for example, seven grid assemblies to eight grid assemblies is being performed. The apparatus must be compatible with both eight grid and seven grid type assemblies, while minimizing the amount of parasitic material placed in the core. Further, it should be adaptable to existing refueling techniques. It should also minimize any effect on the manufacturing process so as not to unduly increase the cost of nuclear fuel and hence power generation.